1. Field of the Invention
The invention relates to a personal watercraft (also called a PWC) that planes along the surface of the water, and more particularly to an exhaust pipe of the personal watercraft and a connecting structure thereof.
2. Description of the Related Art
Personal watercraft have been gaining popularity in recent years for recreational and sports purposes. Generally, the personal watercraft is constructed to thrust forward by increasing the pressure of water drawn through a water intake provided on a bottom of the hull or body of the watercraft, by a propulsion pump, and ejecting the water rearward from the body.
In personal watercraft the exhaust pipe is conventionally provided on a transom board of the body to discharge an exhaust gas rearward of the transom board from an exhaust port of an engine mounted in the body (Japanese Utility Model Publication No. Hei 7-39758).
Furthermore, some personal watercraft are configured so that a rear end of the exhaust pipe is provided with a valve-shaped shut-off member made of rubber which can be opened and closed by opening force born of an exhaust gas pressure in order to reduce exhaust noises (Japanese Unexamined Utility Model Publication No. Hei 5-34100).
In use of personal watercraft, generally, a rider in the water gets on a deck from a rear-end (transom board side) of the body If the above-mentioned shut-off member is provided, the rider is not directly exposed to the exhaust gas stream even when the engine is kept in an operating state.
However, there is a drawback that the exhaust gas pressure in the exhaust pipe is increased by the shut-off member to lower an output of the engine. Also, in a state in which the output of the engine is high, the exhaust gas hitting against the shut-off member is returned to the transom board of the body. Consequently, the exhaust gas makes an external wall of the transom board dirty.
Exhaust pipelines of some personal watercraft are configured so that plural pipe members are connected to each other along the route of the exhaust pipelines. In a connecting portion, generally, a flange is formed on each of the ends of the pipe members to be connected to each other. A seal member such as a packing is provided between the two flanges to be connected to each other. Bolts are inserted through holes provided in corresponding positions of a peripheral portion of each of the two flanges, and fastened, thereby performing the connection with a seal function when the two flanges are pressed against each other. With such a connecting structure of the exhaust pipe members, the mutual alignment (centering) of connecting pipe members, that is to say, the alignment for causing axial centers of respective inner holes (passages) of the pipe members to correspond to each other, is mainly performed in connection with the bolts being inserted through the holes in the respective flanges. However, the size of each hole is generally greater than an outside diameter of the bolt inserted through it. Therefore, the alignment cannot be carried out with precision.
In addition, in a case where the exhaust pipeline connected on a rear flow side of the connecting portion contains one or more pipe members that are bent, a back pressure is generated in the connecting portion. Accordingly, the seal member mentioned above is indispensable in the connecting portion for obtaining a good seal. For this reason, a component parts count of the connecting portion is increased. Furthermore, in assembly of the exhaust pipeline it is necessary to perform complicated work using both hands and skill in such a manner that the bolts are inserted through the holes in the flanges, with the seal member to be held between the flanges.
Also, in order to obtain a good seal, it is necessary to enhance the rigidity of the flanges, so as to uniformly compress the seal member provided between the flanges at the periphery of the inner conduit through which the exhaust gas passes. As a result, the flange has been bulky and heavy. This connecting structure is not desirable for personal watercraft, which generally have only a small space for the exhaust pipeline, and requires a light weight. (Japanese Unexamined Utility Model Publication No. Hei 3-128598 discloses related art for such a connecting structure).